In cellular network networks, particularly wide band cellular networks, traffic for a cell may be carried at a first carrier frequency f1 on a carrier referred to as an underlay carrier typically available in all the cells. In some cases a cell has one or more additional carriers referred to as overlay carriers over which the traffic can be transmitted at respective carrier frequencies. Such cells have overlay carrier capabilities might be used for example in “hot spots” where there is a high concentration of traffic, such as shopping centers and schools. The use of overlay carriers allows more calls to be accommodated resulting in an increase in capacity; however, the overlay carriers can be isolated in that neighboring cells do not necessarily make use of overlay carriers having the same carrier frequencies or may not use any overlay carriers at all. HHO (Hard Hand-Off) mechanisms are used for handing off calls assigned to overlay carriers when mobile terminals move from one cell to another cell that does not have the overlay carrier. HHOs can be risky in that calls can be dropped when a mobile terminal moves one cell to another. SHO (Soft Hand-Off) does not involve switching between carrier frequencies, and is a safer mechanism than HHO in terms of risk of call drop. Therefore, the increase in capacity due to the overlay carrier is accompanied by an increase in a call drop rate.
One approach referred to as a cell cluster approach has been used in which cells neighboring a central cell that covers a hot spot are also provided with overlay carrier capabilities to reduce the call drop rate for calls associated with mobile terminals exiting the central cell. However, deploying additional overlay carriers in neighboring cells is costly. Furthermore, in such systems, hardware used to implement the overlay carriers in the neighboring cells is under-utilized making such an approach inefficient.
Some solutions have been proposed for an isolated overlay carrier having a second carrier frequency f2 in which there is no capability of transmitting traffic on the second carrier frequency f2 for neighboring cells. In one solution, for each call a RTD (Round Trip delay) is measured, and which of the underlay carrier or the overlay carrier is used for the call depending on the RTD. In particular, if the RTD is greater than a threshold RTD the call is assigned to the underlay carrier and if the RTD is less than the threshold RTD the call is assigned to the overlay carrier. With such a method there can still be a large call drop rate if the threshold RTD is large. To reduce the called drop rate the RTD threshold must be reduced thereby reducing the area over which traffic can be transmitted on the overlay carrier. This results in ineffective use of the isolated overlay carrier.
RTD reports are only an approximate indicator of distance due to measurement error from various sources. Calls may also drop in an RTD-only based mechanism because the cell shape (as defined by the area in which the central cell provides adequate signal quality) may not be circular. Furthermore, at its best RTD is a measure of the distance that radio signals travel but this only corresponds to geographic distance if there is a line-of-sight signal path. An RTD-only solution does not account for load variations.
Another method used for isolated overlay carriers requires a deployment of a cluster of pilot beacons in neighboring cells. However, this requires additional hardware and is costly. Furthermore, this method may be intolerant to load variations. Neighbor cell pilot beacons involve transmitting from the neighbor cells a constant power-level on an overlay frequency. With such a construction, a mobile terminal on the central overlay can measure the signal strength of its own carrier in comparison to that of the neighboring cells. The ratio of signal strengths can be used as a trigger for hard hand-off. In addition, the measured pilot strength can assist the mobile terminal selecting a hard hand-off target as it leaves the overlay cell.
Single carrier systems will generally be able to perform soft hand-off. Multi-carrier systems generally have one or more carriers on which soft hand-off is always available, the so-called underlay carriers, and one or more carriers (the so-called overlay carriers) that may require hard hand-off in some instances, for example at the edge of a cluster of cells serviced by the overlay carrier.